Optoelectronic devices interact with radiation and electric current. Such devices can be light-emitting devices that produce radiation as a result of an applied electric voltage/current or photovoltaic (PV) devices, e.g., solar cells that produce an electric voltage/current as a result of applied radiation. Optoelectronic devices typically one or more active layers of photoactive material sandwiched between two electrodes. At least one of the electrodes is transparent. The active layer typically includes two materials exhibiting complementary charge transfer (e.g., one is an electron accepting/transporting material and the other is a hole-accepting/transporting material). In the case of a PV device, at least one of the two materials is a light-absorbing material. In an organic solar cell, radiation absorbed by the active layer creates an exciton (an electron-hole pair) at an interface between the two semiconductor materials. Holes and electrons diffuse through the two different materials such that electrons are collected at one electrode and holes are collected at the other. Unfortunately, electrons and holes can recombine before they are collected, which tends to limit the efficiency of a PV device.
Recently, organic materials, such as gels, conjugated polymers, molecules, and oligomers, have been used as photoactive materials. Random blends of fullerenes and hole-accepting polymers have also been used in organic PV cells. However, these random blends were lacking in order. To increase the efficiency of optoelectronic devices it is desirable to configure the active layer such that the presence of hole accepting and electron accepting materials alternates on a scale of length comparable to the exciton diffusion distance. This distance is typically on the order of several nanometers. To optimize efficiency of the active layer, it is desirable for the arrangement of the hole-accepting and electron-accepting materials to exhibit features of regular shape, uniform size and uniform distribution. These features give excitons a high probability of splitting into electrons and holes, which can migrate to each of their respective electrodes before recombining in the bulk material.
The performance of prior art PV cells is often sub-optimal for one or more reasons. For example, it would be desirable to keep electrons on one side of the active layer and holes on the other, so they cannot recombine before the electrons are pulled out of the device to generate electricity. It would also be desirable for the active layer to absorb light over a broader range of wavelengths than is currently available in a single material. In addition, it would be desirable to enhance light absorption and/or charge injection from the light-absorbing material to the electron-transporting material.
Thus, there is a need in the art for an active layer for an optoelectronic device that overcomes the above disadvantages and a corresponding method of making such an active layer.